Organic compounds are brought into an excited state by absorbing light. Through this excited state, various reactions (photochemical reactions) are caused in some cases, or luminescence is generated in some cases. Therefore, the organic compounds have a wide range of applications.
As one example of the photochemical reactions, a reaction of singlet oxygen with an unsaturated organic molecule (oxygen addition) is known. Since the ground state of an oxygen molecule is a triplet state, oxygen in a singlet state (singlet oxygen) is not generated by direct photoexcitation. However, in the presence of another triplet excited molecule, singlet oxygen is generated to cause an oxygen addition reaction. In this case, a compound capable of forming the triplet excited molecule is referred to as a photosensitizer.
As described above, for generation of singlet oxygen, a photosensitizer capable of forming a triplet excited molecule by photoexcitation is needed. However, the ground state of an ordinary organic compound is a singlet state; therefore, photoexcitation to a triplet excited state is forbidden transition and generation of a triplet excited molecule is difficult. A compound that can easily cause intersystem crossing from the singlet excited state to the triplet excited state (or a compound that allows the forbidden transition of photoexcitation directly to the triplet excited state) is thus required as such a photosensitizer. In other words, such a compound can be used as the photosensitizer and is useful.
Such a compound often exhibits phosphorescence. Phosphorescence refers to luminescence generated by transition between different energies in multiplicity. In an ordinary organic compound, phosphorescence refers to luminescence generated in returning from the triplet excited state to the singlet ground state (in contrast, fluorescence refers to luminescence in returning from the singlet excited state to the singlet ground state). Application fields of a compound capable of exhibiting phosphorescence, that is, a compound capable of converting the triplet excited state into luminescence (hereinafter, referred to as a phosphorescent compound), include a light-emitting element including an organic compound as a light-emitting substance.
This light-emitting element has a simple structure in which a light-emitting layer including an organic compound that is a light-emitting substance is provided between electrodes. This light-emitting element attracts attention as a next-generation flat panel display element in terms of characteristics such as being thin and light in weight, high speed response, and direct current low voltage driving. A display device including this light-emitting element is superior in contrast, image quality, and has wide viewing angle.
The emission mechanism of a light-emitting element in which an organic compound is used as a light-emitting substance is a carrier injection type. That is, by applying voltage with a light-emitting layer interposed between electrodes, electrons and holes injected from the electrodes recombine to make the light-emitting substance excited, and light is emitted when the excited state returns to a ground state. As in the case of photoexcitation described above, types of the excited state include a singlet excited state (S*) and a triplet excited state (T*). The statistical generation ratio thereof in the light-emitting element is S*:T*=1:3.
In a compound which converts a singlet excited state into light emission (hereinafter referred to as a fluorescent compound), light emission from a triplet excited state (phosphorescence) is not observed at a room temperature but only light emission from a singlet excited state (fluorescence) is observed. Therefore, the internal quantum efficiency (the ratio of the number of generated photons to the number of injected carriers) of a light-emitting element including the fluorescent compound is assumed to have a theoretical limit of 25%, on the basis of S*:T*=1:3.
In contrast, in the case of a light-emitting element including the phosphorescent compound described above, the internal quantum efficiency thereof can be improved to 75% to 100% in theory; namely, the emission efficiency thereof can be 3 to 4 times as much as that of the light-emitting element including a fluorescent compound. For this reason, light-emitting elements using a phosphorescent compound have been recently under active development so that light-emitting elements with high efficiency can be achieved. As the phosphorescent compound, an organometallic complex that contains iridium or the like as a central metal has particularly attracted attention because of its high phosphorescence quantum yield (see, for example, Patent Document 1, Patent Document 2, and Patent Document 3).